Saturday, 1 March 2014

Micrograph of the Month: the woods decay and fall


Close-ups of decaying waterlogged wood. Upper left showing 1. latewood with secondary cell wall intact 2. earlywood with loss of secondary cell wall. Upper right showing loss of birefringence in areas subject to decay

Another fab example of the sorts of things that go on in medieval waterlogged deposits. I posted some pictures of this waterlogged wood a couple of months ago, as well as a nice example of the formation of vivianite in the same deposits. Here we have another example of waterlogged wood, but I've added some close-ups in cross polarised light (XPL). For the non geoarchaeologists in the audience, this is a technique in microscopy where you change the type of light you use to look at a sample, by inserting polarisers on the microscope. Polarised light vibrates only in one direction. On the geological microscope, the lower polariser causes the light to vibrate in an E-W direction, whilst the upper polariser/analyser filters light that is not vibrating in the N-S direction.

In non-crystalline specimens such as opal phytoliths and charred wood, this normally means that everything would go black, as no light will pass through. However, when crystalline materials are present, they split the light in such a way that when it reaches the analyser, it causes interference, and passes through instead of being filtered. This splitting of the light  is due to an optical property called birefringence, which means the material has a refractive index that depends on the polarization of light. Minerals tend to be highly birefringent, and can be identified on the basis of colour changes when they are viewed under the XPL. However there are also biological materials which are crystalline, such as cellulose.

In the micrographs you can see here, the images on the left show the wood under plane polarised light, and on the right is the same view in XPL. Initially I thought that this was partial burning of the wood as there are areas which are completely black, however after a bit of research, I think what we may have is  fungal rot in this wood. Because the sample is waterlogged, some of the cellulose has preserved, hence the birefringence (shiny!).  This is a typical pattern, where the cellulose of the secondary cell wall in the earlywood part of the tree ring decays first, followed by the latewood. I suspect a chat with a conservator might be able to help - any waterlogged wood specialists reading?

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